Certain industrial operations have need for processing huge volumes of high temperature pressurized particulate-laden gasses in a manner conserving the heat content thereof while en route to a power recovery gas expander. This need poses serious problems which continue to plague designers for this industry. Various separator designs have been proposed but all are subject to premature failure as well as frequent and costly maintenance and service operations involving very costly interruptions in operations required to be carried out on an uninterrupted continuous basis.
Typical U.S. Pat. Nos. showing these prior proposals include Coward 2,862,571; Dygert et al 2,941,621; Bjorklund 2,986,278; Beins et al 3,066,854; Wilson 3,415,042; Wilson 3,541,766 and Wilson 3,631,657. Most of these separators, as is true of this invention, are intended for use in the petroleum cracking art and in the coal and oil shale pyrolysis art, where the operating conditions are unusually severe owing to the high temperature and pressures and huge volumes of gas required to be processed with minimum heat loss, high efficiency and reliability and on a continuous uninterrupted basis. Typically, such separators operate at 30-40 psig and at 1,200.degree. to 1,300.degree. F. to supply clean gas to gas expanders currently having rated outputs of 30,000 horsepower and now being proposed with a rating of 50,000 horsepower. Not only is the medium being processed highly erosive but its pressure and temperature conditions are subject to wide-range and almost instantaneous process upset changes. These conditions arise when the normally oxygen-deficient gas reaches stoichiometric conditions due to a lowering of the carbon or an increase of the oxygen components within the gas. This upset is known in the industry as "after burn" and can cause gas temperature to rise as high as 2,500.degree. and 2,600.degree. F. with flame-front suddenness. This condition can prevail throughout the separator or, on occasion, may occur in some relatively isolated zone of the separator. The resulting sudden changes in temperature and pressure impose very severe stresses on the structure and cause severe warpage, expansion and rupture of welded joints, and failure of brittle materials such as the ceramic tubes confined by close clearance metal securements commonly used in previous designs.
Stainless steel satisfies substantially all requirements including the high temperature and erosion resistant factors, and could therefore be used for the entire structure including the outer housing. In this case the highly essential conservation of the process heat dictates use of a heat insulating jacket on the exterior of the housing. This poses serious assembly, securement and maintenance problems as respects the insulating jacket to leave unmentioned the prohibitively high cost. Owing to these considerations it has been the practice to employ a carbon steel housing protected by a thick internal insulating lining to conserve process heat and prevent the housing temperature rising above about 325.degree. to 350.degree. F. All other internal metal components unavoidably are normally subject to a temperature of 1,200.degree. to 1,300.degree. F. but some or all may, at times, be subject to 2,500.degree. to 2,600.degree. F. during "afterburn" operating conditions.
The serious stress conditions to which a composite carbon steel-stainless steel structure is subjected by the above mentioned temperature and pressure conditions is evident from the fact that the commonly used 304 type stainless steel expands 12.25 inches per hundred feet at 1,000.degree. F., whereas a similar length of carbon steel expands only 8.8 inches under these conditions. However, under a typical prevailing internal operating temperature of 1,200.degree. F. and a corresponding housing temperature of 350.degree. F., the stainless steel expands 14.7 inches per hundred feet whereas the carbon steel housing expands only 2.3 inches per hundred feet to provide an expansion ratio of 6.4 to 1. The serious aspects of this very high differential expansion become all the more evident when it is realized that this differential expansion ratio can approach 7.5 to 1 during suddenly occuring "afterburn" conditions when the internal temperature of the stainless steel components can reach 1,600.degree. F. without a substantial increase in the temperature of the carbon steel housing.
Various techniques have been proposed for minimizing the stresses in welded joints which are the locale of most structural failures due to process upsets. These include the use of non-flanged dish shaped partitions as exemplified in the aforesaid Wilson patents, concentric inner and outer chambers, expansion joints both interiorly and exteriorly of the main separator housing, various arrangements of internal ducting and manifold structures, and other expedients, all of which are subject to serious shortcomings avoided by this invention.